The invention relates to a base element simultaneously serving the purpose of a movement plate and a case back for a wristwatch.
Base elements of this type are already known and normally consist of a simple piece of material, the outside of which is constructed such that it can serve as the case back of a watch and the inside of which is worked such that it can form the movement plate or frame of the watch movement. The advantage of such a base element forming both the case back and the movement plate is that it enables extremely flat wristwatches to be made, in particular expensive watches intended as pieces of jewelry, in which case the base element is made entirely of a precious metal such as gold. The disadvantage of the known concept is, however, the fact that it is relatively complicated and costly to work the inside, in particular, of the base element, which is later to serve as movement plate. For example, the customary openings in the movement plate, which would normally be simply stamped or drilled, have to be milled out with difficulty and great precision.
Proceeding on the basis of this prior art the object underlying the invention is to propose an improved base element of the type under consideration, which is comparatively easy and cheap to produce even with movement plate elements of high precision.
This object is accomplished according to the invention by a base element of the type described at the beginning, which is characterized by two integrally joined plates.
The basic idea behind the present invention may, therefore, be seen in the fact that, first of all, two relatively simple constructional parts are quickly and cheaply produced using normal procedures and normal apparatus and that these two constructional parts are then joined together to form an integral constructional element which forms the combined base element simultaneously serving the purpose of a case back and a movement plate or frame. Furthermore, the plate members may be of different materials, each suited to its best purpose.
The two plates may, to begin with, be joined together purely mechanically in a positive connection in that, for example, tongues integrally formed with one of the constructional elements are wrapped around the other constructional element to form an integral base element.
It has, however, proven to be particularly advantageous to have the two plates of the base element according to the invention bonded together. It has proven to be an especially favorable method of bonding to have a plastics foil made from thermoplastic material laid between the two plates and for these two plates then to be bonded with the aid of the plastics foil by using pressure and heat, preferably by means of ultrasonic heat-sealing. In this way the bonding between the two plates is very even and occurs over the entire surface. The foil may also have corresponding spaces for the openings in the two plates, in particular in the inner plate, so that later, according to the thickness of the material used for the inner plate, the full height of these openings may be utilized for elements of the watch movement.
Concerning the manufacture of the plates themselves it has proven advantageous to have these made as parts stamped from sheet material, in particular high-quality stainless steel, since stamped metal parts of this type can be produced relatively cheaply but still with high precision. The outer plate for expensive watches intended as pieces of jewelry can also be produced from a relatively thin sheet of gold or other precious metal. If desired, the inner plate can be stamped from less expensive material, such as brass.
If the outer plate is manufactured as a stamped metal part it is also favorable to have these parts made with openings, for example for the battery and screw apertures for screwing the base element and bezel together, such that a collar surrounds the relevant opening on the inside of the outer plate. This collar can later serve to center the inner plate and, if necessary, the foil when the base element is assembled.
Finally, the construction of the base element from two separately manufactured plates offers favorable possibilities of providing attachment elements on these plates at manufacture for later attaching watch straps.